Apparatus for limiting the rate of rise of current in a multi-loop motor control system

ABSTRACT

Described is a system for limiting the rate of rise of current in a multi-loop cascaded motor control configuration by means of a ramp function generator connected to the output of a motor controller, for example, a speed controller, whose output acts as a reference to a current loop.

United States Paten [15] 3,668,495 Eisele et al. [451 June 6, 1972 [54]APPARATUS FOR LIMITING THE RATE References Cited F RISE OF CURRENT IN AMULTI-J. UmD Sums PATENTS LOOP MOTOR Co 7 SYSTEM 3,376,478 4/ 1968Sheng... ..323/9 [72] lnventors: Hermann Ellele, Pittsburgh, Pa.; ColinE; 3,284,688 11/1966 k- --3l /332 Hum, Torrance, n; 3,037,157 /1962Young ..3l8/400 v 3,088,064 4/1963 Anger ..3l8/400 [73] AssigneesWestinghouse Electric Corporation, Pitt- 3,163,812 12/1964 Greening318/400 sburgh, Pa. 3,366,861 1/ 1968 Dudler ..3l8/345 [221 Film PrimaryExaminer-Bemard A. Gilheany Assistant Examiner-Thomas Langer Attorney-F.H. Henson, R. c. Brodahl and c. J. Paznokas [52 us. or 118/400, 323/9 RA[51] Described is a system for limiting the rate of rise of current insfll'ch /3 1, 400, a multi-loop cascaded motor control configuration bymeans 318/332,.345 of a ramp function generator connected to the outputof a motor controller, for example, a speed controller, whose outputacts as a reference to a current loop,

15 Claims, 4

24 32 as 34 36 'SCR 2 SPEED I CURRENT FIRING CONTROLLER CONTROLLERC'RCU'T 4| LIMIT CIRCUIT v RAMP FUNCTIO N GENERATOR 42 I BIAS 25 VOLTAGECl RCU IT PATENTEDJUH 6 m2 3. 668,495

SHEET 10F 2 I8 32 30 33 34 36 2 SPEED I. CURRENT SCH CONTROLLERCONTROLLER 4| CIRCUIT LIMIT CIRCUIT -44 (4O RAMP FUNCTION GENERATOR BIASVOLTAGE CIRCUIT I F LTER f 55 v I 24 CURRENT CONTROLLER FIG.2.

for the motor, for example APPARATUS FOR UMITING THE RATE OF RISE OFCURRENT IN A MULTl-LOOP MUIOR CONTROL SYSTEM BACKGROUND OF Tl-IEINVENTION v Multi-loop cascaded motor conu-ol systems include aplurality of condition regulating loops cascaded toward the controllerpower source'for the motor. The position in the cascade of any looprelative to another loop may be defined as inner or outer using themotor power source as the reference point. For example if a system hasthree loops X, Y and Z, cascaded in that order toward the motor powersource with loop 2 nearest the power source, then loops Y and Z areinner loops with respect to loop X, loops X and Y are outer loops 1 5relative to loop 2, loop Z is inner with respect to loop Y, and loop Xis outer relative to loop Y. Thus in any system having two or morecascaded loops, the positions of any two loops relative to each other isdefined by referring to one as an inner loop and the other as an outerloop. In the above example, loops X, Y and 2, may also be referred to asoutrnost, intermediate and inmost loops, respectively.

The conditions (variables controlled by the various loops may be forexample speed, armature current, motor supply voltage, troller whichusually responds to the difi'erence between the actual and desiredvalues of the particular condition regulated by that loop. The desiredvalue is represented by a reference signal, while the actual value isrepresented by a feedback signal. The transfer functions of thecontrollers-may be pro portional, Pl (proportional integral), PllD(proportional integral derivative), or other depending on the dictatesof the desired dynamics of the system. The output of the controller ofan outer loop is usually applied as a reference to the controller of theadjacent inner loop. An example of a cascaded multi-loop motor controlsystem is described in U.S. Pat. No. 3,324,363. r 1

. The current loop may be the inmost loop, in which case it directlycontrols the power source for the motor. In another case, the currentloop is not the inmost loop, and its output is applied as a reference tothe inmost loop whose output in turn corresponding lead term in thespeed loop. There exists clearly defined limits to the lag term thatrepresents the inner current loop. If the response of the current loopis reduced beyond this limit, then the speed loop must be slowed downalso, resulting in a loss of response, or the control system will ceaseto be linear in operation and will become oscillatory. Thus, slowingdown the response of the inner current loop is not an altogethersatisfactory way of limiting the rate of rise of current in a multi-loopcascaded control configuration.

By using a plurality of amplifiers for the current controller wherein aproportional amplifier feeds into an integrating amplifier, a veryefiective rate of rise of current limit can be obtained. Uponapplication of a large step reference to the pro portional amplifier,the output of this amplifier feeding the integral amplifier willimmediately reach its limit value, and the output of the integrator willstart increasing. Under these conetc. Each condition regulating loopincludes a condirectly controls the power source. in either case thepower switched converter.

Multi-loop cascaded motor control systems often include an outer speedloop having a controller, whose output is fed as a reference signal'intothe controller of an inner current loop. The current controller,responsive to current flowing through the motor, can be used to bycontrolling the firing circuits for thyristors employed to control powerto the motor, or it can feed into a more inner voltage controller which,in turn, regulates the firing circuits.

In any motor control system of this type, the primary objective is toprovide speed and torque control. However, a very important objectiveresides in the provision of means for limiting the rate of rise ofcurrent positive-going or negative-going change) through the motorwindings. Normally, the speed and current controllers are provided withmaximum current limits above which or below which the current cannot go.These limits, however, do not control the rate of rise of current (i.e.,di/dt); and if the rate'of rise of current is too fast in response to astep input or sudden risein required torque, damage to the motor due toflashover at the motor brushes and other causes may result.

In the past, various systems have been proposed to limit the rate ofrise of current supplied to drive motors. One of these involves slowingdown the rate of response of the inner current loop of a .multi-loopsystem. In order to maintain a fast speed of response in an outer speedloop of such a system, the

directly control the power source ditions, one of the two currentfeedback loops will be open, and the system will behave as a singleloop. During this time, the output of the integrating amplifier isbuilding up to its reference value. Close to this value, the output ofthe proportional limit amplifier will decrease; and the currentapproaches its final value without overshoot. This method, however, isdifficult to adjust properly; and when it is adjusted for low rate ofrise of current, the system can get into a limit cycle type of operationdue to wind-up of the speed controller.

SUMMARY OF THE INVENTION As an overall object, the present inventionseeks to provide a motor control system employing means for limiting therate of current change, which system eliminates the disadvantage ofprior art systems for accomplishing the same function.

More specifically, an object of this invention is to provide a motorcontrol system incorporating a rate of current change limit wherein theoutput of a controller is applied to a ramp function generator, and theoutput of the .ramp function generator utilized to gradually increasethe limiting value of the controller. in this manner, the limited outputof the controller, which is relatively low, is gradually increased bythe ramp function generator in response to a step'input or a sudden risein torque, for example, thereby preventing an excessive rate of currentrise.

In accordance with one embodiment of the invention, a motor controlsystem is provided of the type in which a condition controller, forexample a speed controller, responsive to a reference signal and thecondition feedback, is utilized to control the current supplied to amotor, and wherein the output of the controller is limited between upperand lower maximum values. The rate of change of the output of thecontroller is limited by means including a ramp function generatorconnected to the output of the controller and adapted to produce agradually increasing output in response to step input. Circuitry iscoupled to the output of the ramp function generator and adapted inresponse to controller input change to gradually vary the upper andlower limits of the controller either up or down, depending on thedirection of input change, whereby the output of the controller will atfirst move to its limiting value, followed by a gradual change in thatlimiting value in the particular polarity-going direction dictated bythe direction of input change, while the current through the motorchange in the appropriate polarity-going direction as the output of theramp function generator changes along the ramp ftu-iction.

Preferably, the controller is a speed controller which feeds into acurrent controller; however the invention is not limited to thatparticular embodiment, it being understood that the 'control feature ofthe invention can be applied equally well to FIG. 1 is a schematicdiagram of a motor control system slower response of the current loopmust be compensated by a constructed in accordance with the teachings ofthe invention;

FlG.. 2 is a schematic circuit diagram of the speed controller, rampfunction generator, bias voltage circuit and limiting circuitry inaccordance with one embodiment of the invention;

FIG. 3 is a graph illustrating the operation of the present invention;and

FIG. 4 is a schematic circuit diagram of still another embodiment of theinvention.

With reference now to the drawings, and particularly to FIG. 1, anelectric motor is shown connected to a source of alternating currentsupply voltage 12 through a dual converter, symbolically illustrated byforward and reverse thyristor elements 14 and 16. As will be understood,the thyristor element 14, in the usual three-phase alternating currentenergized supply system, represents six separate thyristors which arefired in a prescribed pattern for the forward mode of operation of themotor 10. Similarly, the thyristor element 16 represents six separatethyristors which are operative for the reverse mode of operation of themotor. Thyristor-switched dual converters and the techniques for firingthe thyristors for forward and reverse motor direction are well knownand need no further description.

In the one example of a motor control system, the speed of the motor iscontrolled and regulated by feedback loops which are connected tocontrol the power which the thyristors supply to the motor via thyristorfiring circuit 18. In a typical case, there are three such loops. Anoutermost speed loop regulates and responds to a reference and feedbackfrom the motor supplied through a tachometer generator or othermotor-speed responsive device; an intermediate current loop regulatesresponsive to a reference supplied by the speed loop and negativefeedback supplied through current transformers or other means whichtransmit a signal proportional to the current supplied to the motor; andan innermost voltage loop regulates responsive to a reference suppliedby the current loop and negative feedback dependent on the voltagedirectly controlling the motor, for example, the net armature voltage ofthe motor.

- Although a voltage regulating loop cascaded between the currentloopand the motor power source is desirable, it is not necessary to theoperation or for an understanding of the invention. Thus to simplify theillustration, only an outer speed loop 20 and an inner current loop 22are shown in FIG. 1.

The outer speed loop includes a speed controller 24 and a tachometergenerator 26 connected to the motor and adapted to produce an outputsignal proportional to the speed of the motor. This signal is appliedthrough filter 27 and line 28 as negative feedback to'the input of thespeed controller 24 which may for example be a proportional plusintegral controller. Also applied to the speed controller 24 and summedwith the negative feedback signal from tachometer generator 26 atsumming point 30 is a speed reference signal on lead 32 which, forexample, may be adjusted manually by an operator for a desired motorspeed.

The output of the speed control 24 (on line 33), in turn, is applied asa current reference to the summing input 34 of a current controller 35in the current regulating loop 22 which also may for example be aproportional plus integral controller. The output 36 of the currentcontroller 35, in turn, controls the SCR firing circuit 18. A signalproportional to motor armature current is applied, through a line 37, asnegative feedback to the input of controller 35. The current feedbacksignal may be derived from a resistor 38 in series with the motorarmature and applied'to line 37 through an amplifier 39. Y

ln-accordance with the present invention, the output of the speedcontroller 24, in one embodiment of the invention, is applied through aline 41 to a ramp function generator which controls a bias voltagecircuit 42. The bias voltage circuit42, in turn, is connected to controla limit circuit 44 which, in effect, comprises a part of the speedcontroller 24.

The details of the ramp function generator 40, the bias voltage circuit42, the limit circuit 44 and the speed controller 24 are shown in FIG.v2. The speed controller 24 includes an operational amplifier 46 having atransfer impedance feedback path 48 including a capacitor 50 and aresistor 52 in series. This feedback network gives the amplifier aproportional plus integral transfer characteristic. in shunt with thefeedback path 48 are a limiter 54 and a limiter 56 which are part of thelimit circuit 44 for providing limiting action.

The limit circuit 44 limits the output V of amplifier 46 to voltagesless positive than the voltage V on the input line 53 of limiter 56 andless negative than the voltage V on the input line 55 of limiter 54. Assoon as the output voltage V going in a positive direction, reaches thevalue of voltage V NPN transistor T1 starts to conduct since thepositive voltage V on its base is at least equal to the bias voltage V,,on its emitter. This supplies base current to transistor T2 whichbecomes conductive and now supplies a positive current to the summingjunction of amplifier 46. The positive current prevents a furtherincrease of the amplifier output voltage since the summation of thepositive feedback and the reference signals from leads 32 and 28produces a zero input to amplifier 46. Transistors T3 and T4 work in asimilar fashion if the output V of the amplifier tries to go morenegative than voltage V That is, when the negative voltage on the baseof PNP transistor T3 is at least equal to the bias voltage V on itsemitter, it will conduct.

The speed reference signal on lead 32 is applied to the summing input ofthe operational amplifier 46 through resistor 58; and, similarly, thespeed feedback signal online 28 from tachometer generator 26 is appliedas negative feedback to the summing input of amplifier 46 throughresistor 60.

The output of the operational amplifier 46 is applied via lead 41 to theinput of the ramp function generator 40. As shown, the ramp functiongenerator includes a pair of terminals 64 and 66 adapted for connectionto the positive and negative terminals of a source of direct currentdriving potential, not shown, having its center point grounded.Connected between the terminals 64 and 66 are two current paths, thefirst of which includes a diode bridge network 68 in series with, andintermediate, two transistors 70 and 72. The collector of transistor 70,which is a PNP transistor, is connected to the anodes of two diodes 74and 76 in the bridge 68. Similarly, the collector of transistor 72,which is an NPN transistor, is connected to the cathodes of diodes 78and 80 in the bridge 68. The emitter of transistor 70 is connected tothe positive input terminal 64 through resistor 82; while the emitter oftransistor 72 is connected to the negative input terminal 66 throughresistor 84.

In parallel with the current path just described is a voltage dividernetwork comprising resistors 86, 88, and 92. The junction of resistors86 and 88 is connected to ground through resistor 94; while the junctionof resistors 90 and 92 is connected to ground through resistor'96. Thesignal from the output of operational amplifier 46 is applied to thejunction of diodes 76 and 80 through the input line 41 of the rampfunction generator 40; while a capacitor 98 is connected between thejunction of diodes 74 and 78 and ground. The ramp output of the rampfunction generator is derived across capacitor 98.

As will be understood, the two transistors 70 and 72 are used asconstant current sources. Furthennore, the voltages across the twoemitter resistors 82 and 84 and, therefore, the currents l and I throughthe transistors are determined by the two resistor dividers 86, 88 and90 and 92. That is, the base of transistor 70 is connected to thejunction of resistors 86 and 88, while the base of transistor 72 isconnected to the junction of resistors 90 and 92. This causes the baseof transistor 70 to be biased negative with respect to its emitter andthe base of transistor 72 to be biased positive with respect to itsemitter.

Under steady-state conditions, and assuming that the output of theoperational amplifier 46 is constant, all four diodes in the bridge 68will conduct; and the output voltage across capacitor 98 will be equalto the input voltage. However, if a positive voltage step is applied tothe input (i.e., between 1 diodes 76 and 80), diodes 76 and 78 becomereverse biased since the output voltage across capacitor 98 cannotchangeinstantaneously. Capacitor 98 now charges at a constant rate throughdiode 74-by current 1,, which is kept constant by the emitter-followertransistor 70. The capacitor 98 continues to charge until the outputvoltage across the capacitor 98 again matches the input voltage appliedto the junction of diodes 76 and 80.

If a negative voltage step is applied to the inputof the ramp functiongenerator, the diodes 74 and 80 become reverse biased and capacitor 98is discharged at aconstant rate by current I This discharge willcontinue until the output voltage across the capacitor 98 is again equalto the input voltage. Thus, the ramp function generator 40 provides ameans for converting a stepped input voltage into a ramp functionoutput.

Reverting again to the operational amplifier 46, a change in output willchange the input voltages V V and V on the limiters 54 and 56. Limiter54 limits the output of amplifier 46 to values more positive than thelimiter input voltage V from divider 102 and 106 and limiter 56 allowsonly an output voltage from amplifier 46 which is more negative than thelimiter input voltage V from divider 100 and 104. Whenever the outputvoltage V of amplifier 46 tries to exceed the band determined by theinput voltages to limiters 54 and 56 the respective limiter becomesconductive and presents a very low impedance path parallel to thefeedback path 48 comprising capacitor 50 and resistor 52; and theintegrating effect of the operational amplifier is terminated with theoutput remaining constant. Normally, in the absence of a change in theoutput of the ramp function generator 40, the bias V and V on the inputsof limiters 54 and 56 and, consequently, the point at which theyconduct, is determined by means of a voltage divider connected betweenterminals 64 and 66 and comprising resistors 100 and 102 in series withZener diodes 104 and 106. Connected in shunt with the voltage dividerjust described is a second voltage divider comprising NPN transistor 108having its collector connected to the positive temrinal 64 throughresistor 110 and its emitter connected to the negative terminal 66through resistor 112. The emitter of transistor 108 is connected to thejunction of Zener diodes 104 and 106 as shown.

If it is assumed, for example, that the output of the ramp functiongenerator 40 appearing across capacitor 98 increases in the positivedirection, the voltage on the emitter of transistor 108 will alsoincrease in the positive direction. This increases the positive bias onthe inputs of limiters 54 and 56 and, accordingly, increases thepositive-going output limit while simultaneously increasing thenegative-going output limit in the positive direction, thus translatingupward the position of the output range or band of controller 24.

The operation of the circuit can best be understood by reference to FIG.3 wherein a step input applied to lead 32, for example, is indicated bythe reference numeral 114. For the steady-state time t,, when the stepincrease occurs, the upper and lower output limits of controller 24 areindicated by the broken lines 1 l6 and 118. Upper limit" is the limitfor positive-going output, and lower limit is the limit fornegativegoing output. These limits remain stationary as long as no stepinput is applied to the input of the speed controller. However, at timeI when the step input occurs, the output of controller 24 jumps to itslimiting value established by the level 116. At the same time, thissignal, when applied to the ramp function generator 40, causes a gradualbuildup in voltage across the capacitor 98. As a result, the upper andlower limits increase positively, i.e., move upwardly, along the lines120 and 121, respectively, as shown in FIG. 3 until a new steady statecondition is reached. Conversely, a negative-going input will producethe opposite result. That is, if the step input should increase in thenegative direction rather than the positive direction, then the upperand lower output limits will increase negatively, i.e., they will movedownwardly rather than upwardly as shown in FIG. 3.

Another embodiment of the invention for use in the system of FIG. 1 isshown in FIG. 4 wherein elements corresponding to those shown in FIGS. 1and 2 are identified by like reference numerals. Here, again, areference signal on lead 32 is applied to the summing input of anoperational amplifier 46 through resistor 58; while a feedback signal online 28 from tachometer generator 26 (not shown in FIG. 4) is applied tothe same input through resistor 60. The amplifier 46 is again providedwith a feedback loop including capacitor 50 and resistor 52 whereby itforms a proportional plus integral con troller. In this case, however,the output of amplifier 46 is applied to the output terminal 33 of thecontroller 24 through resistor and either one of the emitter-followertransistors l 16 or 118, depending upon the polarity of the outputsignal.

The emitters of transistors 116 and 118 are connected in common to theoutput terminal 33. These transistors, which act as impedance matchingdevices, have their collectors connected as shown to positive andnegative sources of potential through resistors 120 and 122,respectively. It should be understood that while desirable for someapplications, the impedance matching unit 123 consisting of transistors116 and -1 18 is not necessary to the operation or the understanding ofthe invention, and therefore may be omitted from the circuit. In thelatter case points X and Y should be connected together, thus connectingthe right end of resistor l 15 directly to the output line 33 ofcontroller 24.

The output of amplifier 46 is connected through diode 124 to the outputof operational amplifier 125 having a feedback path 126 includingcapacitor 127 and resistor 128 to provide a proportional plus integralfunction. Similarly, the output of amplifier 46 is connected throughdiode 130 to the output of operational amplifier 132 having a feedbackpath 134 including capacitor 136 and resistor 138 to provide aproportional plus integral function. The input to operational amplifier125 is connected to a movable tap on potentiometer 140 connected betweenground and a source of negative potential. Similarly, the input tooperational amplifier 132 is connected to a movable tap on potentiometer142 connected between ground and a source of positive potential. As willbe seen, the potentiometers 140 and 142 establish the quiescent orminimum limiting values of current in the positive and negativedirections.

If it is assumed, for example, that the output of amplifier 46 shouldincrease in the positive direction, diode 124 will become biased in theforward direction, clamping the rate of change of the output at a valueestablished by the setting on potentiometer 140. At the same time, theincrease in positive potential on terminal 33 is fed back through lead41, capacitor 127 and resistor 128 to the input of amplifier 125,causing its output potential to increase gradually along a ramp due tothe integrating function of capacitor 127. This process will continueuntil a new steady-state condition is reached. At the same time, theoutput terminal 33 increases along a ramp since the clamping level ofdiode 124 is gradually increased. The same action occurs with amplifier132, except in response to negative-going output signals from amplifier46. In the latter case, the output, being negative, blocks diode 124 butcauses diode 130 to become forward biased. At the same time, the outputsignal is fed back via lead 41 to cause the negative clamping level ofdiode 130 to progressively increase.

From the foregoing, it can be seen that the action of the circuit ofFIG. 4 is essentially the same as that of the circuit of FIG. 2 with theupper and lower circuit limits for the motor 10 gradually increasingpositively as the output from controller 24 increases positively, andgradually increasing negatively as the output from controller 24increases negatively.

In both embodiments of FIGS. 2 and 4, since the output of controller 24is the reference for the current controller 35 of the system of FIG. 1,the current through motor 10 follows the output of controller 24,thereby providing limits to the rate of change of motor current inaccordance with the limits of rate of change of the output of thecontroller 24 as hereinbefore described. This applies to bothnegative-going change and positive-going change.

In the circuit of FIG. 4, only the integrating characteristic providedby capacitors 127 and 136 is necessary to ramp function generation. Thusresistors 128 and 138 may be omitted from the feedback paths 126 and134. However, the proportional characteristics imparted by theseresistors may be utilized to compensate for lags in the rest of thesystem. if the resistors 128 and 138 are omitted, the proportional partof the transfer characteristic of amplifiers 125 and'l32 is omitted.This will eliminate the initial jump of controller 24 output to thelimiting value in response to step input. Instead, in response to a stepinput, the output of controller 24, starting from a steady-state valuewill change along the ramp function due to the integratingcharacteristics provided to amplifiers 125 and 132 by the feedbackcapacitors 124 and 130. The output change of controller 24 along a rampfunction will be either positive-going or negative-going, depending onthe direction of input change. The rate of change in motor current willfollow the rate of change in the output of controller 24, and will besubject to the same rate of change limitation.

Although the invention has been shown in connection with certainspecific embodiments, it will be readily apparent to those skilled inthe art that various changes in form and arrangement of parts may bemade to suit requirements without departing from the spirit and scope ofthe invention.

We claim as our invention:

. 1. In a motor control system of the type in which a contrbllerresponsive to a reference signal is utilized to control the currentsupplied to a motor, and wherein the output of the controller is limitedbetween upper and lower maximum values; the improvement of means forlimiting the rate of rise of current at the output of the controllercomprising: a ramp function generator connected to the output of saidcontroller and adapted to produce a gradually increasing output inresponse to a step input, and circuit means coupled to the output ofsaid ramp function generator and adapted to gradually vary the upper andlower limits of said controller whereby the output of the controllerwill at first move to its limiting value followed by a gradual increasein that limiting value while the motor current increases as the outputof said ramp function generator increases.

2. The motor control system of claim 1 wherein said controller is of theproportional plus integral type.

3. The motor control system of claim 1 wherein said controller is aspeed controller.

4. The motor control system of claim 1 wherein said controller isprovided with a transfer impedance feedback path between its input andoutput terminals at least one conduction control means in shunt withsaid feedback path, and means coupled to said ramp function generatorfor controlling said conduction control means to provide limitingaction.

5. The motor control system of claim 4 which includes first and secondconduction control means, each connected between the input and outputterminals of said amplifier, and including means responsive to theoutput of said ramp function generator to cause one of said conductioncontrol means to conduct to provide controlled limiting of negativepolarity output when the ramp output of said generator is negativegoingand for causing the other of said conduction control means to conduct toprovide controlled limiting of positive polarity output when the outputof said ramp function generator is positive-going.

6. The motor control system of claim 5 wherein each of said conductioncontrol means is a limiter for a different polarity output.

7. The motor controller of claim 1 wherein said controller comprises anoperational amplifier having its output connected through a resistor tothe output line of the controller, a transfer impedance feedback pathconnecting the output line of the controller to the input of saidamplifier, said ramp function generator comprising a second operationalamplifier having its input connected to a source of fixed potential, acircuit path comprising a capacitor connecting said output line to saidinput of the second operational amplifier, and a diode connecting theoutput of said second operational amplifier to said output line wherebysaid diode will clamp the output of said first operational amplifier ata level established by said fixed potential source, the clamping levelincreasing along a ramp in response to a step input applied to saidcontroller.

8. The motor controller of claim 1 wherein said controller comprises anoperational amplifier having its output connected to the base side ofemitter-follower transistor means, a transfer impedance feedback pathincluding a capacitor and a resistor in series connecting the emitterside of said emitterfollower transistor means to the input of saidamplifier, said ramp function generator comprising a second operationalamplifier having its input connected to a source of fixed potential, acircuit path including a capacitor connecting said input of the secondoperational amplifier to the emitter side of said emitterfollowertransistor means, and a diode connecting the output of said secondoperational amplifier to the base of said emitter-follower transistormeans whereby said diode will clamp the output of said first operationalamplifier at a level established by said fixed voltage source, theclamping level increasing along a ramp in response to a step inputapplied to said controller.

9. The motor controller of claim 8 wherein said ramp function furthercomprises a third operational amplifier having its input connected to asource of fixed potential, and a second circuit path including acapacitor connecting said input of the third operational amplifier tothe emitter side of said emitterfollower transistor means, and wherein asecond diode connected between the output of the third operationalamplifier and the base side of said emitter-follower transistor meansand adapted to conduct current in one direction, and wherein said diodeconnecting the base side of said emitter-follower transistor means tothe output of said second operational amplifier is adapted to conductcurrent in the opposite direction.

10. The motor controller of claim 9 wherein said emitterfollower meanscomprises a PNP transistor and an NPN transistor having their bases andemitters interconnected, means connecting said bases to the output ofsaid first-mentioned operational amplifier, and means connecting saidemitters to the second and third operational amplifiers.

11. A controllable power supply means connected to the motor forsupplying current to the motor comprising:

A. a motor;

B. first controller means responsive to a first reference signal and afirst condition of said motor for regulating said condition;

C. second controller means responsive to the output of the firstcontroller means and a second condition of said motor for regulating thesecond condition;

D. ramp function generating means coupled to the output of the firstcontroller means for producing a ramp function output in response to astep input into the controller; and

E. adjustable means coupled to the first controller for limiting theoutput of the first controller, said adjustable means being adjustablein response to the ramp function generating means to change said limitvalue as a function of the changing values of said ramp function,whereby a limit is imposed on the rate of change of said secondcondition.

12. The combination as in claim 11 wherein the second condition is motorcurrent.

13. The combination as in claim 12 wherein the first condition is motorspeed.

14. The combination as in claim 11 wherein said ramp function generatingmeans produces a ramp function sloping in one direction in response topositive-going step input into the first controller means, and producesa ramp function sloping in the opposite direction in response tonegative-going step 1 input into the first controller means, and whereinsaid adjustable means responds (a) to said one direction slope rampfunction to limit the rate of change of said second condition in onedirection, and (b) to said opposite direction slope ramp function tolimit the rate of change of said second condition in the oppositedirection.

15. The combination as in claim 11 wherein:

1. In a motor control system of the type in which a controllerresponsive to a reference signal is utilized to control the currentsupplied to a motor, and wherein the output of the controller is limitedbetween upper and lower maximum values; the improvement of means forlimiting the rate of rise of current at the output of the controllercomprising: a ramp function generator connected to the output of saidcontroller and adapted to produce a gradually increasing output inresponse to a step input, and circuit means coupled to the output ofsaid ramp function generator and adapted to gradually vary the upper andlower limits of said controller whereby the output of the controllerwill at first move to its limiting value followed by a gradual increasein that limiting value while the motor current increases as the outputof said ramp function generator increases.
 2. The motor control systemof claim 1 wherein said controller is of the proportional plus integraltype.
 3. The motor control system of claim 1 wherein said controller isa speed controller.
 4. The motor control system of claim 1 wherein saidcontroller is provided with a transfer impedance feedback path betweenits input and output terminals, at least one conduction control means inshunt with said feedback path, and means coupled to said ramp functiongenerator for controlling said conduction control means to providelimiting action.
 5. The motor control system of claim 4 which includesfirst and second conduction control means, each connected between theinput and output terminals of said amplifier, and including meansresponsive to the output of said ramp function generator to cause one ofsaid conduction control means to conduct to provide controlled limitingof negative polarity output when the ramp output of said generator isnegative-going and for causing the other of said conduction controlmeans to conduct to provide controlled limiting of positive polarityoutput when the output of said ramp function generator ispositive-going.
 6. The motor control system of claim 5 wherein each ofsaid conduction control means is a limiter for a different polarityoutput.
 7. The motor controller of claim 1 wherein said controllercomprises an operational amplifier having its output connected through aresistor to the output line of the controller, a transfer impedancefeedback path connecting the output line of the controller to the inputof said amplifier, said ramp function generator comprising a secondoperational amplifier having its input connected to a source of fixedpotential, a circuit path comprising a capacitor connecting said outputline to said input of the second operational amplifier, and a diodeconnecting the output of said second operational amplifier to saidoutput line whereby said diode will clamp the output of said firstoperational amplifier at a level established by said fixed potentialsource, the clamping level increasing along a ramp in response to a stepinput applied to said controller.
 8. The motor controller of claim 1wherein said controller comprises an operational amplifier having itsoutput connected to the base side of emitter-follower transistor means,a transfer impedance feedback path including a capacitor and a resistorin series connecting the emitter side of said emitter-followertransistor means to the input of said amplifier, said ramp functiongenerator comprising a second operational amplifier having its inputconnected to a source of fixed potential, a circuit path including acapacitor connecting said input of the second operational amplifier tothe emitter side of said emitter-follower transistor means, and a diodeconnecting the output of said second operational amplifier to the basEof said emitter-follower transistor means whereby said diode will clampthe output of said first operational amplifier at a level established bysaid fixed voltage source, the clamping level increasing along a ramp inresponse to a step input applied to said controller.
 9. The motorcontroller of claim 8 wherein said ramp function further comprises athird operational amplifier having its input connected to a source offixed potential, and a second circuit path including a capacitorconnecting said input of the third operational amplifier to the emitterside of said emitter-follower transistor means, and wherein a seconddiode connected between the output of the third operational amplifierand the base side of said emitter-follower transistor means and adaptedto conduct current in one direction, and wherein said diode connectingthe base side of said emitter-follower transistor means to the output ofsaid second operational amplifier is adapted to conduct current in theopposite direction.
 10. The motor controller of claim 9 wherein saidemitter-follower means comprises a PNP transistor and an NPN transistorhaving their bases and emitters interconnected, means connecting saidbases to the output of said first-mentioned operational amplifier, andmeans connecting said emitters to the second and third operationalamplifiers.
 11. A controllable power supply means connected to the motorfor supplying current to the motor comprising: A. a motor; B. firstcontroller means responsive to a first reference signal and a firstcondition of said motor for regulating said condition; C. secondcontroller means responsive to the output of the first controller meansand a second condition of said motor for regulating the secondcondition; D. ramp function generating means coupled to the output ofthe first controller means for producing a ramp function output inresponse to a step input into the controller; and E. adjustable meanscoupled to the first controller for limiting the output of the firstcontroller, said adjustable means being adjustable in response to theramp function generating means to change said limit value as a functionof the changing values of said ramp function, whereby a limit is imposedon the rate of change of said second condition.
 12. The combination asin claim 11 wherein the second condition is motor current.
 13. Thecombination as in claim 12 wherein the first condition is motor speed.14. The combination as in claim 11 wherein said ramp function generatingmeans produces a ramp function sloping in one direction in response topositive-going step input into the first controller means, and producesa ramp function sloping in the opposite direction in response tonegative-going step input into the first controller means, and whereinsaid adjustable means responds (a) to said one direction slope rampfunction to limit the rate of change of said second condition in onedirection, and (b) to said opposite direction slope ramp function tolimit the rate of change of said second condition in the oppositedirection.
 15. The combination as in claim 11 wherein: G. saidadjustable limiting means comprises an operational amplifier, a fixedvoltage source connected to the amplifier input, and a diode connectingthe output of the first controller means and the output of theamplifier, and H. said ramp generating means comprises said amplifierand feedback path including a capacitor connecting the output of thefirst controller to the input of the amplifier.